This application, entitled, "Molecular Determinants of Leptin Receptor/Jak2 Signaling," is a revised competitive renewal of DK56731. The long (or LRb) isoform of the leptin receptor mediates signaling and the physiologic action of leptin to regulate energy balance (decrease feeding and increase energy expenditure) and neuroendocrine function. It is not clear why leptin fails to adequately protect from obesity and the predisposition to Type 2 diabetes in humans; it is thus critical to understand the molecular details of LRb signaling in order to understand potential mechanisms of leptin resistance and/or to identify potential targets for the therapy of obesity. The long-term outlook of our previous and future studies is to understand the regulation and mechanisms of signaling by LRb. LRb is a type 1 cytokine receptor that, although devoid of enzymatic activity, mediates phosphotyrosine-dependent signaling by means of an associated Jak2 tyrosine kinase. Although we have already learned a great deal about the biology of LRb action, there have arisen a number of important questions regarding mechanisms of signaling that are mediated by the LRb-associated Jak2 independently of LRb phosphorylation (which we refer to as Jak2-autonomous signals); Jak2-autonomous signals likely mediate important leptin actions in vivo. We hypothesize that phosphorylation sites on Jak2 regulate Jak2 activity as well as mediating Jak2-autonomous signals during LRb signaling. The focus of this application is to understand Jak2-autonomous signals and regulatory inputs to Jak2 during LRb activation by defining phosphorylation sites within Jak2 and their roles in LRb action in cultured cells and in vivo. In addition to shedding light upon the biology of LRb specifically, our analysis will illuminate basic mechanisms of cytokine receptor/tyrosine kinase signaling. We propose to: Specific Aim 1: Identify Jak2 phosphorylation sites. Specific Aim 2: Define the function of Jak2 phosphorylation sites in LRb signaling. Specific Aim 3: Determine the phosphorylation and function of Jak2 during leptin signaling in vivo. Each of these aims is crucial to addressing the central hypothesis of this proposal- that Jak2-autonomous signals and Jak2 phosphorylation are critical to the regulation and propagation of physiologic leptin action.